X-ray and UV Dual Photochromism, Thermochromism, Electrochromism, and Amine-Selective Chemochromism in an Anderson-like Zn₇ Cluster-Based 7-Fold Interpenetrated Framework

TLDR

Smart materials are increasingly sought after for applications such as smart windows and sensors, yet stable, stimuli‑responsive viologen‑based systems remain scarce. The study aims to enhance chromic sensitivity by incorporating polynuclear metal‑carboxylate clusters as electron donors into a stable framework. A novel viologen polymer featuring a 7‑fold interpenetrated Anderson‑like Zn7 cluster and a 3D pcu network was synthesized, using bpybc ligands and Zn7SBU nodes as 6‑connected building units. The material exhibits exceptional multi‑stimuli chromism—including soft X‑ray/UV dual light, temperature, electricity, and amines—alongside core‑to‑edge thermochromic switching, high thermal/chemical stability, and enhanced repeatability when incorporated into a hybrid film, indicating promise for future smart devices.

Abstract

Smart materials are highly desirable over the recent decade due to the growing demand of complicated nature. Stable stimuli-responsive smart materials exhibit widespread potential for applications in smart windows, sensors, separators, chemical valves, and release platforms but are rare. Despite being good candidates, viologen-based multifunctional smart materials are still a challenging task for chemists. To obtain such materials, the judicious strategy is to introduce polynuclear metal-carboxylate clusters as electron donors into a stable framework to increase chromic sensitivity. Toward this endeavor, we have synthesized a novel viologen-based polymer with a unique Anderson-like metal-carboxylate cluster, [Zn7(bpybc)3(o-BDC)6]·2NO3·6H2O (bpybc = 1,1'-bis(4-carboxyphenyl)-4,4'-bipyridinium, o-BDC = o-benzenedicarboylic acid) (1), which is a particular 7-fold interpenetrated framework with a 3D pcu network in which bpybc ligand as the linker and Zn7O30C12 as the second building unit (Zn7 SBU) were used as 6-connected nodes. More importantly, it shows excellent chromic behavior in response to multiple external stimuli especially soft X-ray and UV dual light, temperature, electricity, and organic amines, which stand out in the viologen-based polymers. Interestingly, the coloration process of 1 from "core" to "edge" is observed upon heating at the appropriate temperature, which has not yet been found in other reported thermochromic materials. Of particular interest for 1 is the couple of quaternary stimuli-sensitive abilities because it simultaneously meets the following conditions: (i) the capability of withstanding high light, higher temperature, extreme pH, and other harsh conditions; and (ii) the high sensitivity to external stimuli keeping away from photodegradation, thermal relaxation, side reactions, and so on. To be noted, 1 has high thermal stability and chemical stability, which are excellent advantages as smart materials. To further develop possible practical utilization, 1 has been doped into the polymer matrixes to construct a hybrid film, which not only keeps the response to external stimuli but also significantly improves the repeatability of the photochromic process, indicating that a new smart device with multi-stimuli-responsive functions will emerge successively in the future.